The Montblanc manufacturer's guarantee requires proof. What is intended for the purpose is the guarantee card in the service booklet, filled in and stamped by an authorised dealer. If both the stamp and a proof of purchase are missing, there is no claim under the guarantee. If an invoice at least is available, the chances are considerably better; that is no guarantee in itself, though, since with a pen not obtained through an authorised dealer or a boutique the manufacturer may refuse to honour the guarantee. In practice the decision often goes in the customer's favour, but it is not something to rely on.
This must be kept apart from statutory warranty rights: these are owed by the seller, including a commercial seller online. Anyone buying online from a trader should therefore always insist on a proper invoice and keep it together with the service booklet. In private sales the warranty is routinely excluded, leaving only the manufacturer's guarantee, and then only where it can be evidenced. For second-hand buying this means that a completed service booklet and a proof of purchase are a solid factor in the value of a pen, not a formality.